Power meters are frequently used for measuring signal strength at microwave frequencies since the power level of a signal along a lossless transmission line will remain constant, while the current and the voltage components of the signal vary periodically with distance. For convenience in making measurements, the transducer which absorbs the signal to be measured is usually remote from the actual metering circuitry and is connected to the metering circuitry by a flexible cable. The sensitivity of power transducers commonly used with microwave power meters usually varies from unit to unit; and, thus, most power meters provide a front panel adjustment for setting the sensitivity of the metering circuitry to correspond with the sensitivity of the transducer being used in order to achieve calibrated readings. In addition, some power meters can be used with transducers covering different ranges, requiring a change in the meter scale factor when the transducer is changed.
To facilitate the use of transducers with various sensitivities, some power meters have provided coding circuits within the transducer. These coding circuits are connected to the power metering circuitry with a plurality of wires in the cable between the transducer and the power meter to adjust the sensitivity of the metering circuitry according to the code wired into the transducer as well as appropriately adjusting the display. Such display adjustments may comprise lighting different meter scales or adding a constant to a digital display. While this particular system increases operator convenience it is disadvantageous because it increases the cost and bulk of the transducer and cable assembly and requires more expensive connectors due to the increased number of wires and connector pins.
According to a preferred embodiment of the present invention a power transducer is provided with a coding resistor which is connected to the power metering circuit by a single wire. A current is driven through this resistor via the wire, using the gound conductor already present in the cable as a return path for the current. The voltage across the resistor is measured and coded in binary form by an analog-to-digital (A/D) converter circuit for adjusting the sensitivity of the metering circuit. The A/D converter uses a series of feed-forward comparators in which the output of a previous comparator is used to adjust the reference level against which the voltage across the resistor is compared. Since the A/D converter is static and free-running, it does not require a clock or elaborate chains of feedback comparators, thus reducing the cost and complexity of the system.